Disaggregation of Revenue - Schedule of Disaggregation of Revenue (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Disaggregation of Revenue [Line Items]
Revenue	$ 354,048	$ 365,424	$ 343,020	$ 335,208	$ 336,230	$ 345,558	$ 338,861	$ 326,738	$ 1,397,700	$ 1,347,387	$ 1,303,937
Flex revenue
Disaggregation of Revenue [Line Items]
Revenue									1,364,097	1,299,687	1,258,249
Direct Hire revenue
Disaggregation of Revenue [Line Items]
Revenue									33,603	47,700	45,688
Tech
Disaggregation of Revenue [Line Items]
Revenue									1,049,628	1,057,859	990,089
Tech | Flex revenue
Disaggregation of Revenue [Line Items]
Revenue									1,032,901	1,037,380	971,310
Tech | Direct Hire revenue
Disaggregation of Revenue [Line Items]
Revenue									16,727	20,479	18,779
FA
Disaggregation of Revenue [Line Items]
Revenue									348,072	289,528	313,848
FA | Flex revenue
Disaggregation of Revenue [Line Items]
Revenue									331,196	262,307	286,939
FA | Direct Hire revenue
Disaggregation of Revenue [Line Items]
Revenue									$ 16,876	$ 27,221	$ 26,909